Yes, it is a builtin, but:
10] perldoc -f glob
glob In list context, returns a (possibly empty)
list of filename expansions on the value of
EXPR such as the standard Unix shell /bin/csh
would do. In scalar context, glob iterates
through such filename expansions,
returning undef when the list is exhausted.
This is the internal function implementing
the "<*.c>" operator, but you can use it directly.
If EXPR is omitted, $_ is used. The "<*.c>"
operator is discussed in more
detail in "I/O Operators" in perlop.
Beginning with v5.6.0, this operator is
implemented using the standard "File::Glob"
extension. See File::Glob for details.
So the builtin glob calls load the File::Glob module on the fly apparently